index.js

// Original implementation:
// http://lolengine.net/blog/2014/02/24/quaternion-from-two-vectors-final

var dot = require('gl-vec3/dot')
var set = require('gl-vec3/set')
var normalize = require('gl-quat/normalize')
var cross = require('gl-vec3/cross')

var tmp = [0, 0, 0]
var EPS = 1e-6

module.exports = quatFromUnitVec3
function quatFromUnitVec3 (out, a, b) {
  // assumes a and b are normalized
  var r = dot(a, b) + 1
  if (r < EPS) {
    /* If u and v are exactly opposite, rotate 180 degrees
     * around an arbitrary orthogonal axis. Axis normalisation
     * can happen later, when we normalise the quaternion. */
    r = 0
    if (Math.abs(a[0]) > Math.abs(a[2])) {
      set(tmp, -a[1], a[0], 0)
    } else {
      set(tmp, 0, -a[2], a[1])
    }
  } else {
    /* Otherwise, build quaternion the standard way. */
    cross(tmp, a, b)
  }

  out[0] = tmp[0]
  out[1] = tmp[1]
  out[2] = tmp[2]
  out[3] = r
  normalize(out, out)
  return out
}